This invention relates to manufacture of plastic articles and more particularly relates to pneumatic conveyance and processing of plastic resin pellets prior to molding or extrusion of those pellets into a finished or semi-finished plastic product.
In this patent application, injection and compression molding presses and extruders are collectively referred to as “process machines.”
The Technology
The plastics industry is very diversified; there are thousands of different products, hundreds of materials, and dozens of processes, and all are very different from one another. The only thing all these differences share in common is that the source material is some type of plastic.
Equipment sold to this industry is, therefore, very diverse in design. Plastics factories have multiple process machines, sometimes several hundred in one location. Virtually all plastics fabricating operations require that each process machine, namely a molding press or an extruder, be supplied automatically with the required raw resin material on a continuous basis. This resin may be supplied in large boxes, called Gaylords, in fiber drums, in 50 pound bags, or more typically may be delivered by bulk truck or rail car, with the resin material then being transferred in bulk into storage silos. In all cases the resin material must be further distributed throughout the plant to each and every process machine. For that reason a great deal of design and capital expense is devoted to the automatic distribution of the raw resin material throughout the plant.
These resin distribution systems, more commonly referred to as “Loading Systems”, must deal with many variables. One set of variables includes the type, shape, size and consistency of the granular material.
Resin pellets, nominally about ⅛ inch in size, come in various shapes, with round, square, and cylindrical being the most common.
Flowing resin powder is also an option, and very fine but free flowing resin pellets and other granular materials may be conveyed as well,
The design variables to be considered for each customer include:                1. Type of resin being conveyed.        2. Size and consistency of the resin pellets.        3. Distances the resin pellets are to be conveyed.        4. Variability of these distances from shortest to longest.        5. Acceptable range for velocity of resin material travel through the lines.        6. Throughput rate of resin required for each machine.        7. Total throughput rate of resin for the entire plant.        8. Excess capacity performance margin so a molding or extrusion process is not interrupted by short term loading issues.        9. Loss of resin material from or at the supply so that only air is being pulled, thereby reducing system vacuum levels and reducing overall design throughput.        10. Loading sequence, or priority, when multiple receiver stations call for material.        11. Detecting problems and alarm conditions.        12. Proper air to material ratio for resin conveying.        13. Detecting plugged lines due to poor resin flow or over feeding of resin material.        14. Dust condition and filter requirements.        15. Reliability.        16. Serviceability.        17. Ease of use.        18. Cost        19. Vacuum pump type, namely positive displacement, regenerative, and others.        20. Vacuum pump horsepower and rated CFM capacity as well as vacuum levels.        
In all of these areas, system designers look to find improved methods and solutions whenever possible.
One of the most important considerations is to hold a correct velocity for the conveyed resin material. The type of resin material dictates the target conveying speed. To maximize the resin material transfer rate, a high conveying speed is preferred, and air speed in any case must be sufficient to keep the resin pellets suspended and moving in the air stream. But velocity must be limited so as not to damage the pellets. Hard brittle pellets can fracture and break when conveyed, resulting in excessive dust.
Softer pellets can skid along the conduit walls, causing “angel hair” as a result of the plastic resin melting at the point of high speed contact with the conduit wall; this leaves a thin film on the wall. Strings of very thin “angel hair” accumulate, effectively reducing diameter of the conduit and causing problems in the system.
Air speed and resin conveying velocity are directly related to pump capacity (rated Cubic Feet per Minute, abbreviated as “CFM”) and horsepower, as well as conveying line diameter. There is always a correct velocity “range” for each type of resin material. It is a design challenge to assure that resin material is conveyed within the correct velocity range.
Conveying distances affect system design. Conveying over short distances requires a less powerful vacuum source then over longer distances. Systems are generally sized to produce the best compromise for material velocity between the shortest and longest conveying distance.
Required conveying rate usually dictates line size (tube diameter), and this in turn dictates the CFM required to maintain correct velocity in a given diameter conduit. This means different tube sizes in the same system can be a problem if one vacuum pump is to draw air and resin through several different diameter conveying lines. Pumps have known CFM ratings. Pulling air through a small tube will result in higher velocity flow than pulling the same CFM through a larger tube.
Excessive velocity can damage pellets.
The type of vacuum pump to be selected is important. Regenerative blowers deliver wide ranging CFM depending on vacuum level. Positive displacement type pumps deliver high vacuum levels, and have a flatter CFM curve over their vacuum range. Regenerative blowers are quieter and generally cost less. Positive displacement blowers may require sound enclosures and tend to cost more, but are generally more reliable and more forgiving as respecting dust in the air.
The simplest systems use a fixed speed motor to drive the vacuum pump, and a single size conveying line to serve all receivers regardless of distance, rate requirement, or material.
VFD (Variable Frequency Drive) motors allow vacuum pumps to operate at different speeds, and therefore at different CFM rates, with the vacuum pump pulling different vacuum levels depending on preset information about each receiver being served, and/or making adjustments based on real time feedback of vacuum sensors located at various places in the system.
The addition of a SCFM (Standard Cubic Feet per Minute) limiter in the air flow line allows oversized vacuum pumps to be used without risk of conveying at excessive velocity. SCFM limiters restrict air flow to a preset SCFM. This maintains the desired SCFM air flow at the inlet, which is critical for proper conveying for a given size conveying line. This concept is the subject of co-pending parent U.S. patent application Ser. No. 14/185,016, published 20 Aug. 2015 as United States patent publication 2015/0232287 A1.
Reading vacuum levels at various points tells the controlling processor if the line is open, which means only air and no material is present and air is flowing unrestrictedly. This signals a loss of material at the source. A high vacuum reading indicates a plugged or nearly plugged line. Normal conditions are present where material is flowing correctly at detected mid-vacuum levels.
One line size for all receivers assures the resin transport velocity is more likely to be in the acceptable range. However, most processes require the base resin material be delivered at 50 times the rate of additives, such as color concentrate. Virgin (or natural) pellets may have to be loaded at a rate of 1000 pounds per hour, requiring a 2.5 or 3 inch line size, while color is only required to be delivered at a rate of 20 to 40 pounds an hour. A smaller receiver is used for color, namely a receiver one that loads perhaps 5 pounds at a time, while the receiver for the virgin resin material will be larger, perhaps loading 50 pounds of each load cycle. A 2.5 inch line on a 5 pound receiver would be too large. 1.5 inch line would be standard, and the use of 1.5 inch resin conveying line would be better. But this risks velocities that are excessive. This results in trade-offs in design.
By placing a flow limiter at the pump suction intake, one can limit the maximum SCFM air flow to the design limit of the air flow limiter device.